Multi-task setup [DBZ-6984]

Migrated from [DBZ-6984](https://issues.redhat.com/browse/DBZ-6984)

During our load test, we found that currently Debezium mongo connector is capped around 30MB/s-40MB/s due to it's single threaded way of streaming. 

Currently although each connector can work on different DB/table, but it's difficult to solve the hot table issue. With the proposed setup, this can be scaled up easily.

Currently we are planning to do a multi-task setup for mongodb, i.e. three connectors reading from a single replset. Each connector only works 1/3 of the traffic split by hash of time. With this, Debezium throughput can be improved to 100+MB/s.

I'm not sure if
* Debezium community will be interested in this patch for mongo, we will be happy to send this patch out once it's done.
* other databases could apply the similar approach?

Or is there any other solution you have already tried?

Start by reviewing the Debezium MongoDB connector's current single-threaded streaming behavior and the proposed multi-task setup described in the issue. The work is ready when the community agrees on a concrete design for splitting replset traffic across tasks and defines how the approach should be validated against the stated throughput goal.

Written by the indexing model from the issue text.